The present description generally relates to an electric storage device such as a battery, and more particularly relates to thermal control of an electric storage device which supplies power to propel a vehicle and stores power recovered during vehicle deceleration.
Electric systems that include an electric storage device or battery for propelling a hybrid electric vehicle are known and presented, for example in Japanese Patent Application Publication 2005-63689A. In one example, the battery discharges electricity to propel the vehicle. The battery can also be charged with electric power that is recovered during the vehicle deceleration. These discharging and charging cycles are repeated during travel of the vehicle, which can cause a temperature of the battery to increase. When a temperature of the battery is higher, an internal resistance of the battery is lower and the efficiency of charging and discharging is increased.
However, when the battery temperature goes above a certain level, it may cause the battery electrolyte to degrade. As a result, the efficiency of charging and discharging the battery may be reduced. Therefore, it is desirable to keep the battery at an efficient operating temperature that is below the level at which the battery electrolyte degrades. To address this issue, the '689 publication discloses a method of cooling the battery by using a cooling fan driven by an electric motor when the battery temperature exceeds a threshold temperature.
According to the prior art method, by cooling the battery so as to keep the battery temperature below a threshold temperature, the battery charging and discharging efficiency degradation that can occur at the higher battery temperature may be reduced. However, there is still room for improvement. In the prior art method, the threshold temperature to cool the battery is fixed. This can cause the cooling fan to be operated more often than is needed to keep the discharging and charging efficiency during some conditions. On the other hand this can cause the cooling fan to operate less during other conditions, depending on the threshold temperature setting. More energy may be consumed to drive the cooling fan. Or, the battery temperature may increase more than desired, and the efficiency of charging and discharging may be degraded. As a result, the overall efficiency of the electric system may be degraded.